1. Field of the Invention
The inventions disclosed and taught herein relate generally to installing subsea pipelines; and more specifically relate to installing subsea pipelines where the location of an installation vessel is restricted.
2. Description of the Related Art
Deep sea well operations often include the establishment of a subsea station on the seabed, or sea floor, which may serve as a collecting or assembly point for a plurality of subsea devices, such as pipelines. Prior systems exist for installing subsea stations and for coupling components thereto. Some of the prior systems are limited to depths at which divers may be used. Others are directed to diverless applications and propose various systems for accomplishing the positioning and connecting of subsea components. A known procedure is to use a first surface support to lower a subsea system to the seabed. The subsea system has a pulley around which a hauling cable is wound and connected to a winch on the surface support. After placement of the subsea system, a loose end of the cable is hauled by a surface vessel to second surface support holding the pipeline, where the loose end is attached to the pipeline. The winch is activated and pulls the pipeline to the subsea system with the pulley. With this procedure, at least two aspects occur. First, surface access is needed between the different surface supports and the surface vessel to manage the connections between the subsea system, the pipeline, and the hauling cable. Second, catenary forces from the pipeline weight remain on the pulley during the operation as the pipeline is lowered into position. The pulley must be sized to withstand such forces. The construction and sizing is generally considered expensive and necessarily complicated. Likewise, the hauling cable is sized to withstand such forces and further adds to the costs of the operation. In some prior art systems, the hauling cable can be about 65 mm in diameter of more expensive synthetic material and are commonly 3000 meters long or more depending on the depth to the seabed and the distance to travel downward through the pulley and upward to the pipeline.
Another procedure is to lower a sled to the seabed, where the sled is connected to flowlines and contains a detachable frame with a winch. An overhead surface vessel lowers a riser string to the sled and connects to the frame, then lifts out the frame with the winch. The overhead surface vessel then moves into a position overhead of a nearby wellhead and lands the frame on the wellhead. The frame is connected to the sled with cables. The winch on the frame is operated remotely from the surface vessel with umbilical hydraulic lines to pull the sled and hence flowlines to the wellhead. An upper portion of the frame is disconnected, leaving the cables and equipment of a lower portion of the frame down on the wellhead. A series of valves are landed on the lower portion to connect the flowlines to the well. The operation involves overhead access to perform the various connections, requires expense from the various equipment and umbilical cords, and adds the additional complication to operate the winch remotely from the surface.
Often times, subsea stations exist on the seabed below some surface facility, such as a vessel, drilling rig, production facility or the like. When the desire exists to install a pipeline or to otherwise access a subsea station, the surface facility above may restrict access, such as by limiting the positioning of a pipelay or installation vessel. In such a scenario, desired access to the subsea station or other target area from the sea's surface directly above may be restricted or unavailable and the above procedures may be unsatisfactory for such operations, besides having the other previously mentioned disadvantages of known procedures.
Therefore, there remains a need to provide a more efficient system and method for installing subsea equipment.